I don't use carbon personally. I used to, but after a while it seemed like an unnecessary expense. I do have a buddy that uses it in his filters as a "timer" of sorts. Basically, whenever he can smell the filter (when he opens his stand) he knows it is time to clean his filter. I think that by the time it starts smelling is too long, but it works for him. He has a great african cichlid tank with fish breeding all of the time - so I guess he can't be doing anything too wrong.